This invention relates to release liners for normally tacky and pressure-sensitive adhesive, and is particularly concerned with an improved silicone-coated release liner.
For many years it has been common practice to protect the adhesive-coated face of normally tacky and pressure-sensitive adhesive sheet material with a removable release liner. Such liners typically include paper or other base sheet material having on at least one face a release coating to which pressure-sensitive adhesive adheres less firmly than it does to the tape backing on which it is coated. A wide variety of release coatings have been employed, but the lowest release values have typically been obtained when a silicone polymer is employed for this purpose.
In the early development of silicone-coated release liners, an organic solvent solution of relatively high molecular weight, viscous OH-terminated polydimethyl siloxane, polymethylhydrosiloxane crosslinking agent, and metal salt catalyst, such as dibutyl tin dilaurate or lead octoate, was applied to a sheet material and heated to evaporate the solvent and crosslink the polymer through its OH groups, into a tough silicone coating; see, e.g., U.S. Pat. No. 3,061,567. When applied to the adhesive surface of pressure-sensitive adhesive tape, such liners provide low initial release values and maintain satisfactorily low release values even after being in contact with the adhesive for weeks or months. Stable release values are achieved even if the cured silicone surface of the liner has previously been exposed to natural or artificial light under ambient temperature and humidity conditions for substantial periods of time. Such exposure may occur in a drying oven when web breakage or stoppage occurs. It may also occur when the opposite side of the sheet is subsequently coated with silicone in the same manner to provide a two-surfaced release liner.
In more recent times, the evaporation of organic solvent into the atmosphere has been prohibited by air pollution standards, necessitating the installation of expensive scrubbers or solvent recovery systems. As a result, manufacturers have turned to the use of solvent-free reactants in preparing silicone release liners. A 100% solids blend of low molecular weight, low viscosity vinyl-functional polydimethyl siloxane, polymethylhydrosiloxane crosslinking agent, and noble metal hydrosilation catalyst is coated on the base sheet material and thereafter heated to effect cure by crosslinking through the vinyl groups, forming a tough solid silicone coating; see, e.g., U.S. Pat. No. 4,216,252. While this method is less expensive and more convenient than the previous method of preparing silicone release liners, products prepared in this manner (especially liners having silicone coatings on both faces) have suffered from a serious disadvantage, the reason for which was not understood.
Like the liners in which the silicone coating was applied from solvent, liners made with solvent-free silicone systems provide release values which are initially satisfactory. This fact is readily verified by the quality control tests performed by liner manufacturers, where a strip of pressure-sensitive adhesive tape is adhered to a liner using a weighted roller and stripped therefrom within a few minutes under controlled conditions while measuring the force required for removal. Unfortunately, however, the low initial release sometimes--but not always--begins to increase within 30 minutes after the liner is placed in contact with the adhesive, rising sharply to an intolerably high value in an exponential curve over the first week or so. As a result of the unpredictability of performance, liners made with solvent-free silicone systems (especially liners coated with silicone on each face) have been the cause of many customer complaints. Neither the reason for this erratic performance nor any method of coping with it has been recognized.